1. Field of the Invention
The present invention relates to an image forming apparatus which forms an image with toner, such as a copier, a printer, a facsimile machine, a plotter, or a multifunction machine having at least two of the functions of these apparatuses, and to an image forming method employed by such an image forming apparatus. The present invention specifically relates to an image forming apparatus and an image forming method capable of adjusting the density of the image.
2. Description of the Related Art
A so-called electrophotographic image forming apparatus forms an image with toner. Such an image forming apparatus forms an image by causing a charging device to uniformly charge an image carrier, causing an exposure device to form a latent image on the image carrier on the basis of data input in accordance with the image to be formed, and causing a development device to make toner to adhere to the latent image.
In recent years, such image forming apparatuses have spread throughout the printing industry, and with it demand for higher output speed and higher image quality has rapidly been increasing. The demand for higher image quality includes a strong demand for density uniformity within a page, i.e., uniformity in density of an image formed on a recording medium, such as a sheet of paper. Uniform density within a page has become one criterion users use to select an image forming apparatus. It is therefore important to minimize uneven density in a page.
Uneven density is caused by various factors, such as uneven charging of the image carrier due to non-uniform charging, uneven exposure by the exposure device, uneven rotation or uneven sensitivity of the image carrier such as a photoconductor, uneven resistance of a developer carrier such as a development roller, uneven charging of toner, and uneven transfer by a transfer roller, for example. Uneven density attributed to, among other things, the uneven rotation or uneven sensitivity of the image carrier particularly occurs at relatively short intervals, and thus periodically occurs within a page and is easily noticeable. It is therefore particularly important to suppress uneven density attributed to the uneven rotation or uneven sensitivity of the image carrier.
The above-described varieties of uneven density will now be described in detail. Uneven density attributed to the uneven rotation of the image carrier will be described first. In the electrophotographic image forming apparatus, an electric field generated by the potential difference between the developer carrier and the image carrier causes toner to adhere to an outer circumferential surface of the image carrier, to thereby form an image. The uneven rotation of the image carrier, therefore, changes a development gap between the image carrier and the developer carrier, resulting in a change in the electric field and thus a change in image density.
Meanwhile, uneven density attributed to the uneven sensitivity of the image carrier occurs as follows. The sensitivity of the image carrier to the electric field, which affects the toner adhesion amount, changes in accordance with the image density. That is, the type of potential difference determining the sensitivity of the image carrier to the electric field affecting the toner adhesion amount changes in accordance with the image density, thereby changing the sensitivity of the image carrier. Specifically, in a shallow portion having a relatively high density, such as a solid image portion having a relatively large toner adhesion amount, a development potential (i.e., a potential difference between a development bias and a light potential) is dominant. Meanwhile, in a halftone or highlighted image portion smaller in toner adhesion amount than the shallow portion, a background potential (i.e., a potential difference between a development bias and a dark potential) is dominant. Herein, the light potential refers to the potential of an exposed portion of the image carrier, and the dark potential refers to the potential of a non-exposed portion of the image carrier.
If a parameter such as the development bias is controlled to correct uneven density in the shadow portion, therefore, the effect of the control is not obtained in the halftone or highlighted image portion, and uneven density is increased. For example, the development bias may be modulated in accordance with the rotation period of the image carrier, with the use of a rotational position sensor which detects the rotational position of the image carrier and a density sensor which detects the density of the image. According to this method, uneven density detected by the density sensor is divided by the rotation period of the image carrier, and the development bias is periodically changed with the use of the signal of the rotational position sensor as a trigger, such that the change of the electric field due to factors such as the uneven rotation of the image carrier is cancelled to stabilize the electric field and thereby suppress the detected uneven density.
According to this method, however, the effect of the control changes in accordance with the image density, as described above, and thus uneven density may occur in an image having a density different from the image density for which the effect of the control is expected. As previously noted, this is because the sensitivity of the image carrier to the electric field affecting toner adhesion amount changes in accordance with the image density. That is, in an image having a relatively high density, such as a density of 100%, for example, the development potential is dominant, and the modulation of the development bias results in a reduction in uneven density. Meanwhile, in an image having a relatively low density, such as a halftone or highlighted image, in which uneven density is less noticeable and the image density is affected by the background potential, the modulation of the development bias by itself causes a change in background potential, and thus may increase uneven density.
Meanwhile, streaking periodically occurring in an image may be comprehensively reduced by a method employed by an electrophotographic or electrostatic-recording image forming apparatus including a first fluctuation data storage device which previously stores periodical density fluctuation data of the image density and a first controller which controls an image forming condition on the basis of the density fluctuation data. According to this correction method, the first fluctuation data storage device stores density fluctuation data corresponding to at least one rotation period of the developer carrier, and the first controller controls one of the charging voltage, the exposure light amount, the development voltage, and the transfer voltage to correct the density in accordance with the rotation period of the image carrier. This method, however, is not free from the above-described issues.
Uneven density may be suppressed by another method focusing not on the rotation period of the image carrier but on the rotation period of the developer carrier. The method changes the development bias in accordance with the rotation period of the development roller, to thereby reduce uneven density occurring in the image with the rotation period of the development roller. Specifically, the method detects uneven density in image patterns formed on the image carrier, and performs phase matching between the detected uneven density information and the rotation of the development roller, to thereby control the development bias. If the control target is limited to the development bias, however, it is highly possible that the image density correction works in the solid image but not in the halftone image.
As a method addressing the above-described issues, the detection of uneven density may be performed with images formed at different densities, and the development bias and another image forming condition may be controlled to adjust uneven density in the images of the different densities. However, the toner consumption is increased by the formation of the images with different densities, and the load on a cleaning mechanism for cleaning off the formed images is increased. Further, the time taken for the image formation and the cleaning is increased, extending apparatus downtime.
That is, if the method of correcting uneven density due to the uneven sensitivity or uneven rotation of the image carrier involves forming a plurality of image patterns for detecting uneven density and creating correction data for image forming conditions, such as the charging bias, the development bias, and the exposure condition, to change the image forming conditions on the basis of the rotation period of the image carrier, the above-described issues arise.
For example, as the above-described method, the development bias and the charging bias may be modulated in accordance with the rotation period of the image carrier. Specifically, an uneven density detection pattern may be formed in a shadow portion to create correction data for the development bias, and then an uneven density detection pattern may be formed in a halftone portion to create correction data for the charging bias, to thereby reduce uneven density irrespective of the image density. This method also modulates the charging bias, and thus uneven density due to the control is suppressed in the halftone portion in which the background potential is dominant. This method, however, forms the two types of detection patterns described above, and thus causes the above-described increase in the toner consumption, the load on the cleaning mechanism, and the apparatus downtime.
Uneven density attributed to the uneven sensitivity of the image carrier further includes the following type. That is, if the sensitivity of the image carrier to exposure varies owing to factors such as an environmental change or overall deterioration with time, the light potential of the exposed portion of the image carrier varies, even if the image carrier is exposed with constant exposure, and causes a change in the electric field and thus a change in image density, resulting in uneven density. It is therefore preferable to also suppress uneven density due to this factor. To address the uneven sensitivity of the image carrier, a high-precision manufacturing method of the image carrier may be employed to reduce the change in sensitivity. The method, however, leads to an increase in cost, which is to be avoided as much as possible.